It has been previously known to mount a subframe on a vehicle body via resilient bushes, and to pivotally attach the inner ends of the lower arms of a wheel suspension system to the subframe. The subframe typically comprises a pair of laterally spaced longitudinal members, and a plurality of lateral members passed across the longitudinal members. The longitudinal members and the lateral members are normally made by stamp forming sheet metal, and each member is typically provided with a closed cross section to the end of ensuring sufficient rigidity. For an example of such a subframe assembly, reference should be made to Japanese utility model publication (kokoku) No. 61-39651.
Typically, the lower arms of the suspension system are pivotally supported by bearing portions provided in one of the lateral members, and the lateral loads applied by the lower arms of the suspension system are therefore primarily supported by the lateral member.
The longitudinal and lateral members normally consist of sheet metal which is stamp formed and welded into profiled hollow members which have closed cross sections so that sufficient rigidity and mechanical strength can be ensured. Typically, each of the members is fabricated by stamp forming a U-shaped lower half, and an inverted U-shaped upper half, and welding them together by electric spot welding. Therefore, conventionally, it was not possible to integrally form bearing portions in the lateral member, and separate bracket members were therefore welded to the lateral member to form the bearing portions for the lower arms of the suspension system.
However, the brackets serving as the bearing portions are not capable of evenly distributing the load applied to the entire lateral member as they are simply welded to the lateral member. Therefore, the high rigidity and mechanical strength achieved by the closed cross section of the lateral member cannot be fully utilized for supporting the load applied by the lower arms of the suspension system.
It is conceivable to weld the brackets over a large surface area of the lateral member to improve their capability to transmit loads applied thereto, but it would inevitably increase not only the size and weight of the brackets but also the amount of welding work. The increase in the welding work leads to the increase in the effort required to control the quality of welding.